IGCSE CIE Biology: The Nitrogen Cycle | IGCSE CIE 生物：氮循环考点精讲

📚 IGCSE CIE Biology: The Nitrogen Cycle | IGCSE CIE 生物：氮循环考点精讲

Nitrogen is an essential element for all living organisms because it is a key component of proteins, DNA, RNA, and ATP. Even though Earth’s atmosphere is about 78% nitrogen gas (N₂), most organisms cannot use this inert form directly. The nitrogen cycle describes the series of natural processes that convert nitrogen into biologically available compounds, cycle it through ecosystems, and eventually return it to the atmosphere. Mastering the nitrogen cycle is critical for success in IGCSE CIE Biology, as it features frequently in both core and extended tier questions.

氮是所有生物体必需的元素，因为它是蛋白质、DNA、RNA 和 ATP 的关键组分。尽管地球大气中约 78% 是氮气 (N₂)，但大多数生物不能直接利用这种惰性的形式。氮循环描述了一系列自然过程，这些过程将氮转化为生物可利用的化合物，使其通过生态系统循环，并最终将其返回大气层。掌握氮循环对于在 IGCSE CIE 生物学中取得成功至关重要，因为它在核心和扩展层级的试题中都频繁出现。

1. Overview of the Nitrogen Cycle | 氮循环概述

The nitrogen cycle is a biogeochemical cycle that moves nitrogen through the atmosphere, soil, water, and living organisms. The major reservoirs are the atmosphere (gaseous N₂), soil (nitrate ions NO₃⁻, ammonium ions NH₄⁺, and organic nitrogen), and biomass (proteins, nucleic acids). The key transformations include nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. Each step is driven largely by specific groups of microorganisms, making the cycle an excellent example of the role of bacteria in maintaining ecosystem balance.

氮循环是一种生物地球化学循环，使氮在大气、土壤、水和生物体之间循环。主要的储存库是大气（气态 N₂）、土壤（硝酸根离子 NO₃⁻、铵根离子 NH₄⁺ 和有机氮）以及生物量（蛋白质、核酸）。关键的转化过程包括固氮作用、硝化作用、同化作用、氨化作用和反硝化作用。每一步在很大程度上都由特定的微生物群驱动，使得该循环成为体现细菌在维持生态系统平衡中作用的一个极好例子。

2. Why Living Organisms Require Nitrogen | 生物为什么需要氮

Plants and animals need nitrogen to synthesise amino acids, which are the monomers that make up proteins. Nitrogen is also a fundamental part of the nitrogenous bases in DNA and RNA (adenine, thymine, cytosine, guanine, and uracil), as well as chlorophyll and ATP. Without a continuous supply of usable nitrogen, growth and reproduction would be severely limited. Plants absorb nitrogen mainly in the form of nitrate ions (NO₃⁻) from the soil through their roots, although they can take up some ammonium ions (NH₄⁺) as well.

植物和动物需要氮来合成氨基酸，氨基酸是组成蛋白质的单体。氮也是组成 DNA 和 RNA 中含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶、鸟嘌呤和尿嘧啶）以及叶绿素和 ATP 的基础部分。若没有持续的可利用氮供应，生长和繁殖将受到严重限制。植物主要通过根部以硝酸根离子 (NO₃⁻) 形式从土壤中吸收氮，尽管它们也能摄取部分铵根离子 (NH₄⁺)。

In animals, nitrogen is obtained by consuming plants or other animals. Proteins are digested into amino acids, which are then reassembled into the organism’s own proteins. Excess amino acids are deaminated in the liver, and the nitrogen is excreted as urea or uric acid. This excreted nitrogen eventually re-enters the soil and becomes available for decomposers, linking the food chain to the nitrogen cycle.

在动物中，氮通过食用植物或其他动物获取。蛋白质被消化为氨基酸，然后重新组装为该生物体自身的蛋白质。多余的氨基酸在肝脏中经脱氨作用除去氨基，其含氮部分以尿素或尿酸形式排出。这种排出的氮最终重新进入土壤并被分解者利用，从而将食物链与氮循环联系起来。

3. Nitrogen Fixation | 固氮作用

Nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). Because the triple bond in N₂ is extremely strong (N≡N), a large amount of energy is required to break it. In nature, fixation occurs through two main routes: abiotic fixation by lightning and biological fixation by certain bacteria and archaea.

固氮作用是将大气中的氮气 (N₂) 转化为氨 (NH₃) 或铵根离子 (NH₄⁺) 的过程。由于 N₂ 中的三键 (N≡N) 非常牢固，需要大量能量才能使其断裂。在自然界中，固氮主要通过两种途径进行：通过闪电的非生物固氮和通过某些细菌和古菌的生物固氮。

Lightning fixation: The extreme heat of a lightning bolt causes N₂ to react with O₂ to form nitrogen oxides (NO and NO₂). These dissolve in rainwater to produce nitric acid (HNO₃), which falls to the ground and forms nitrate ions (NO₃⁻) that plants can use. Although spectacular, lightning contributes only a small fraction of the total fixed nitrogen.

闪电固氮：闪电极高的温度使 N₂ 与 O₂ 反应生成氮氧化物（NO 和 NO₂）。它们溶解在雨水中形成硝酸 (HNO₃)，落到地面并生成植物可利用的硝酸根离子 (NO₃⁻)。虽然很壮观，但闪电固定的氮仅占固氮总量的一小部分。

Biological nitrogen fixation is far more significant. Free-living soil bacteria such as Azotobacter and Clostridium, and especially symbiotic bacteria like Rhizobium, carry out this process. Rhizobium forms a mutualistic relationship with leguminous plants (e.g., peas, beans, clover), invading their root hairs and inducing the formation of root nodules. Inside these nodules, the bacteria produce the enzyme nitrogenase, which reduces N₂ to ammonia. The equation can be simplified as:

生物固氮则重要得多。自由生活的土壤细菌，如固氮菌和梭菌，以及特别是共生细菌如根瘤菌，执行这一过程。根瘤菌与豆科植物（如豌豆、菜豆、三叶草）形成互利共生关系，侵入其根毛并诱导根瘤的形成。在这些根瘤内，细菌产生固氮酶，将 N₂ 还原为氨。该过程可简化为：

N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂

The plant gains a supply of fixed nitrogen, while the bacteria receive carbohydrates and a protected, anaerobic environment. IGCSE candidates should be able to explain this mutualism and recognise root nodules in diagrams or photographs.

植物由此获得固定氮的供应，而细菌则获得碳水化合物和受保护的厌氧环境。IGCSE 的考生应能解释这种互利共生关系，并在图表或照片中识别根瘤。

4. Nitrification | 硝化作用

Nitrification is a two-step aerobic process carried out by nitrifying bacteria in the soil. It converts the ammonia or ammonium ions produced during fixation or ammonification into nitrites and then into nitrates, which are the preferred nitrogen source for most plants.

硝化作用是由土壤中的硝化细菌进行的两步需氧过程。它将固氮作用或氨化作用产生的氨或铵根离子转化为亚硝酸盐，再转化为硝酸盐，后者是大多数植物首选的氮源。

Step 1 – Ammonia to nitrite: Bacteria such as Nitrosomonas oxidise ammonium ions (NH₄⁺) to nitrite ions (NO₂⁻).

第一步 – 氨转化为亚硝酸盐：如亚硝化单胞菌等细菌将铵根离子 (NH₄⁺) 氧化为亚硝酸根离子 (NO₂⁻)。

2NH₄⁺ + 3O₂ → 2NO₂⁻ + 2H₂O + 4H⁺

Step 2 – Nitrite to nitrate: Bacteria such as Nitrobacter further oxidise nitrite ions to nitrate ions (NO₃⁻).

第二步 – 亚硝酸盐转化为硝酸盐：如硝化杆菌等细菌进一步将亚硝酸根离子氧化为硝酸根离子 (NO₃⁻)。

2NO₂⁻ + O₂ → 2NO₃⁻

The combined process can be memorised using the sequence NH₄⁺ → NO₂⁻ → NO₃⁻. Nitrification is an oxidation reaction that releases energy, which the bacteria use to synthesise organic molecules. Because oxygen is required, nitrification is suppressed in waterlogged or compacted soils, a fact often tested in CIE exams.

整个过程的顺序可以借助 NH₄⁺ → NO₂⁻ → NO₃⁻ 来记忆。硝化作用是一种释放能量的氧化反应，细菌利用该能量合成有机分子。由于需要氧气，在积水或板结的土壤中硝化作用会受到抑制，这往往是 CIE 考试的一个考点。

5. Assimilation | 同化作用

Plants absorb nitrate ions (and to a lesser extent ammonium ions) from the soil solution through their root hairs via active transport. Inside the plant, these ions are used to synthesise amino acids, proteins, nucleic acids, and chlorophyll. This incorporation of inorganic nitrogen into organic molecules is called assimilation.

植物通过根毛以主动运输的方式从土壤溶液中吸收硝酸根离子（以及在较小程度上吸收铵根离子）。在植物体内，这些离子被用于合成氨基酸、蛋白质、核酸和叶绿素。这种将无机氮掺入有机分子的过程称为同化作用。

When herbivores consume plants, they digest plant proteins into amino acids and then reassemble them into their own proteins. Carnivores obtain nitrogen by eating other animals. In this way, nitrogen moves through the food web. Any organic nitrogen in dead organisms or metabolic waste is returned to the soil pool to be processed by decomposers.

当食草动物取食植物时，它们将植物蛋白消化成氨基酸，然后重新组装成自身的蛋白质。食肉动物通过捕食其他动物获取氮。通过这种方式，氮沿着食物网移动。死去的生物体或代谢废物中的任何有机氮都会返回土壤库，由分解者进行处理。

6. Ammonification (Decomposition) | 氨化作用（分解）

Ammonification is the process by which decomposers – mainly saprotrophic bacteria and fungi – break down the organic nitrogen present in dead organisms, faeces, and urine, converting it back into ammonium ions (NH₄⁺). The decomposers secrete extracellular enzymes that digest complex nitrogenous compounds (proteins, urea, nucleic acids) into simpler substances, releasing ammonium ions into the soil in the process.

氨化作用是分解者（主要是腐生细菌和真菌）分解存在于死生物体、粪便和尿液中的有机氮，并将其转化回铵根离子 (NH₄⁺) 的过程。分解者分泌胞外酶，将复杂的含氮化合物（蛋白质、尿素、核酸）消化为较简单的物质，并在此过程中将铵根离子释放到土壤中。

The ammonia produced can then enter the nitrification pathway or, in some soils, be taken up directly by plants. Ammonification is crucial because it recycles the vast amount of organic nitrogen locked up in biomass. Without decomposers, essential nutrients would remain trapped and unavailable for primary producers.

产生的氨随后可进入硝化途径，或在某些土壤中直接被植物吸收。氨化作用至关重要，因为它使锁定在生物量中的大量有机氮得以循环。没有分解者，必需的养分将一直被锁死，无法供初级生产者利用。

7. Denitrification | 反硝化作用

Denitrification is the anaerobic conversion of nitrate ions (NO₃⁻) back into nitrogen gas (N₂), which returns to the atmosphere. This process is performed by denitrifying bacteria such as Pseudomonas and Thiobacillus, which use nitrate as an alternative electron acceptor in the absence of oxygen. The overall reaction can be summarised as:

反硝化作用是在厌氧条件下将硝酸根离子 (NO₃⁻) 转化回氮气 (N₂) 并返回大气的过程。这一过程由反硝化细菌完成，如假单胞菌和硫杆菌，它们在无氧条件下利用硝酸盐作为替代的电子受体。总反应可概括为：

2NO₃⁻ + 10e⁻ + 12H⁺ → N₂ + 6H₂O

Denitrification is favoured in waterlogged, poorly aerated soils and in sediments of lakes and oceans. From an agricultural perspective, denitrification is generally undesirable because it removes plant-available nitrate from the soil, reducing soil fertility. Farmers often drain fields and avoid over-irrigation to minimise this nitrogen loss. This is the main biological pathway by which fixed nitrogen returns to the atmospheric reservoir, completing the cycle.

反硝化作用在积水、通气不良的土壤以及湖泊和海洋的沉积物中更容易发生。从农业角度看，反硝化作用通常是不利的，因为它从土壤中除去植物可用的硝酸盐，降低了土壤肥力。农民们常常给田地排水并避免过度灌溉，以尽量减少这种氮损失。这是固定氮返回大气储存库的主要生物途径，使氮循环得以闭合。

8. Summary of Key Microbial Players | 关键微生物作用总结

The nitrogen cycle is largely driven by four functional groups of bacteria. The table below summarises their roles, products, and oxygen requirements. IGCSE candidates are expected to name examples where specified by the syllabus, and to link each process to the correct bacterial group.

氮循环主要由四类功能细菌驱动。下表总结了它们的作用、产物和对氧气的需求。IGCSE 考生需能根据大纲要求举出例子，并将每个过程与正确的细菌群联系起来。

Bacterial Group 细菌群	Process 过程	Key Products 关键产物	Oxygen Requirement 需氧性	Examples 例子
Nitrogen-fixing bacteria 固氮菌	Fixation 固氮	NH₃ / NH₄⁺	Aerobic / anaerobic 需氧/厌氧	Rhizobium (symbiotic), Azotobacter (free-living)
Nitrifying bacteria 硝化细菌	Nitrification 硝化	NO₂⁻ then NO₃⁻	Aerobic 需氧	Nitrosomonas, Nitrobacter
Decomposers 分解者	Ammonification 氨化	NH₄⁺	Aerobic / anaerobic 需氧/厌氧	Saprotrophic bacteria and fungi 腐生细菌和真菌
Denitrifying bacteria 反硝化细菌	Denitrification 反硝化	N₂	Anaerobic 厌氧	Pseudomonas, Thiobacillus

9. Human Impacts on the Nitrogen Cycle | 人类活动对氮循环的影响

Human activities have significantly altered the global nitrogen cycle, often leading to environmental problems. The Haber-Bosch process industrially fixes atmospheric nitrogen to produce ammonium-based fertilisers. While this has boosted crop yields, it has also doubled the rate at which reactive nitrogen enters the biosphere.

人类活动极大地改变了全球氮循环，往往导致环境问题。哈伯-博斯法工业上固定大气中的氮以生产铵基化肥。虽然这提高了作物产量，但也使活性氮进入生物圈的速率翻了一番。

Excess fertiliser use leads to nitrate leaching into groundwater and surface water. This can cause eutrophication: an overgrowth of algae and aquatic plants that depletes oxygen levels when they die and decompose, resulting in the death of fish and other aquatic life. The CIE syllabus expects students to link nitrate run‑off to eutrophication and to understand how algal blooms create anoxic conditions.

过量施用化肥导致硝酸盐淋失进入地下水和地表水。这可引起富营养化：藻类和水生植物过度生长，它们死亡和分解时耗尽了氧气，导致鱼类和其他水生生物死亡。CIE 大纲要求学生能将硝酸盐径流与富营养化联系起来，并理解水华如何造成缺氧条件。

Burning of fossil fuels releases nitrogen oxides into the atmosphere, contributing to acid rain and respiratory problems. Additionally, vast areas of waterlogged rice paddies and poorly drained agricultural fields favour denitrification, releasing N₂ and the potent greenhouse gas nitrous oxide (N₂O) into the atmosphere. Understanding these links highlights the importance of sustainable agricultural practices such as crop rotation with legumes and controlled irrigation.

化石燃料的燃烧将氮氧化物释放到大气中，导致酸雨和呼吸系统问题。此外，大面积积水稻田和排水不良的农田有利于反硝化作用，释放 N₂ 和强效温室气体一氧化二氮 (N₂O)。理解这些联系凸显了可持续农业实践的重要性，例如与豆科植物轮作和可控灌溉。

10. Key Exam Tips and Common Pitfalls | 常见考点与易错点

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IGCSE CIE Biology: The Nitrogen Cycle | IGCSE CIE 生物：氮循环 考点精讲